The present invention relates to a method of in a liquid separating a mixture of particles that within chosen limits have different physical property values, wherein the particles to be separated are supplied via a feeder to a screen upon which a layer of facilitating particles is disposed, the size of the facilitating particles being too large to fit through the openings in the screen, wherein particles to be separated are passed through the layer of facilitating particles and through the openings in the screen and collected under the screen, the density of the particles being greater than that of the liquid, and wherein a repetitive vertical movement is provided between the particles and the liquid, respectively. Furthermore, the invention relates to a device for carrying out such a method.
Such a method and device are known from the American patent specification U.S. Pat. No. 4,772,384. This patent mentions the separation of very small particles with dimensions up to 1 mm and which are of mineral origin, such as coal, ore and the like. The separation is based only on the differences in density of the particles to be separated. Particles having a highest density will pass through the layer of facilitating particles and the screen downward, while particles having a lowest density will remain behind on the layer of facilitating particles and will be removed from above the same.
In the known method, liquid is fed from below upward through the separating screen. This will push the facilitating particles also upward together with the particles to be separated. When the upward movement is stopped, or even reversed into a downward movement of the liquid, the particles, in particular the heaviest particles, will have occupied the spaces formed between the facilitating particles. After such liquid flow movement has been repeated several times, a considerable portion of the heavier particles will have passed the layer of facilitating particles and the screen. These particles can subsequently be collected under the screen in a suitable receptacle.
A drawback of this known method is that with the heavier particles a considerable portion of the lighter particles will also pass through the screen downward and be collected in the receptacle. This problem is very noticeable especially if there is little difference between the various densities of particles to be separated. Moreover, this known method has only been shown to be useful in practice when separating mixtures comprising heavy to very heavy particles and much lighter particles such as mixtures of coals or ores with contaminants.
Another drawback of this method is that in many cases, simultaneous with the removal of the lighter particles, it is also necessary to remove a portion of the layer of facilitating particles. This results in a more complex process as well as in considerable wear on the facilitating particles.
Finally, this known method has the drawback that two layers of different types of facilitating particles are required on the separating screen: a bottom layer of relatively heavy material that will not be displaced by the liquid, and a top layer of lighter material.